1. Field of the Invention
The present invention relates to an optical fiber and an optical waveguide.
2. Description of the Related Art
In a case of a single-mode optical fiber made of generally-used silica glass, its wavelength dispersion characteristic is expressed by the sum of wavelength dispersion determined based on a refractive index structure of the optical fiber, that is, waveguide dispersion, and wavelength dispersion determined based on optical characteristic of silica glass as a constituent material of the optical fiber, that is, material dispersion. The waveguide dispersion can substantially change the characteristic by changing a refractive-index distribution shape held by the optical fiber. Therefore, a zero-dispersion wavelength in which wavelength dispersion becomes zero can be set to a desired wavelength by adjusting the waveguide dispersion of the optical fiber. However, a wavelength range in which the zero-dispersion wavelength can be easily set by adjusting the waveguide dispersion is 1200 nm or more, and it is difficult to set the zero dispersion characteristic to a wavelength shorter than 1200 nm. The reason for this is explained below. A value of material dispersion held by the silica glass is positive in the wavelength longer than a wavelength of about 1300 nm and expresses anomalous dispersion. However, the positive and negative are reversed in the wavelength of about 1300 nm. In a region of a short wavelength, the value of the material dispersion expresses a large normal dispersion when the wavelength becomes shorter. On the other hand, the waveguide dispersion is basically a normal dispersion in the wavelength larger than about 1000 nm. Even when the wavelength dispersion is shifted to the anomalous dispersion side by changing the refractive-index distribution shape, the absolute value becomes only small. Therefore, the wavelength dispersion as the sum of the material dispersion and the waveguide dispersion of the optical fiber becomes negative. Consequently, it is difficult to set the zero-dispersion wavelength to a wavelength shorter than 1200 nm. Particularly, it is impossible to set the zero-dispersion wavelength to a range of wavelength of 900 nm to 1150 nm as a near-infrared region.
To cope with the problem, in recent years, there has been reported an optical fiber generally called a photonic crystal fiber having many holes formed around a core region made of silica glass. There is a report that a waveguide dispersion having a large absolute value is obtained in the photonic crystal fiber (see, for example, T. A. Birks, et al., “Dispersion compensation using single-material fibers”, Photon. Tech. Lett. 11, 674 (1999) and J. C. Knight, et al., “Anomalous dispersion in photonic crystal fiber”, Photon. Tech. Lett. 12, 807 (2000)). There is also a report that a single mode operation and a zero-dispersion wavelength characteristic can be obtained in an optional wavelength, by using a structure having this hole formed in the photonic crystal fiber (see, for example, P. J. Bennett, et al., “Toward practical holey fiber technology: fabrication, splicing, modeling, and characterization”, Opt. Lett. 24, 1203 (1999)). That is, in the photonic crystal fiber, the refractive index structure of the optical fiber can be substantially changed, by laying out many holes of about 60 to 300 within a cladding region, thereby obtaining waveguide dispersion of a large absolute value. Accordingly, a large anomalous dispersion can be obtained in a near-infrared short-wavelength region, for example. Consequently, the wavelength dispersion can be set to zero, by summing up with the material dispersion having a large normal dispersion. In the photonic crystal fiber, the wavelength dispersion characteristic substantially depends on the sizes of holes and precision of the hole layout. However, because it is difficult to manufacture an optical fiber having many holes laid out in high precision, productivity decreases and cost increases. Further, in the photonic crystal fiber, because a dopant to increase the refractive index of germanium and the like is not added to the core region, effective refractive index is low. As a result, confinement loss of light easily becomes large. To suppress this loss, many hole layers need to be provided. Because, a total number of holes cannot be decreased, productivity further decreases and cost increases.
On the other hand, there is recently reported an optical fiber having a structure called a Hole-assisted Fiber, having holes provided around the core region doped with germanium (see, for example, T. Hasegawa, et al., “Novel hole-assisted lightguide fiber exhibiting large anomalous dispersion and low loss below 1 dB/km”, OFC2001, D5-1). According to this hole-assisted fiber, light can be confined strongly to the core region by providing holes, and therefore, macro-bending loss can be decreased. Further, waveguide dispersion can be substantially changed by providing holes near the core region. Unlike the photonic crystal fiber having many holes provided to suppress confinement loss, the hole-assisted fiber has the core region having a higher refractive index than that of the cladding region. Therefore, the effective refractive index becomes higher than the refractive index of the cladding region. Consequently, confinement loss of light can be easily suppressed, without providing many hole layers.
However, regarding the single-mode optical fiber having a zero-dispersion wavelength in the wavelength of 900 nm to 1150 nm as a waveband of which use has been in high demand recently and in which waveband the optical fiber is properly used, it has been difficult and has been at high cost to manufacture the single-mode optical fiber in high precision when a photonic crystal fiber is used.